ECE4448 L43: Diode-Based Overdrive and Distortion Effects (Guitar Amplification and Effects) 

"When guitarplayers say gain, the really mean distortion. Although this is technically speaking a overdrive pedal and not what usually called a distortion pedal. Although the distinctions are fairly fuzzy". I laughed. Brilliant.

One of very few videos that actually talk physics instead of subjective impressions. Thank you.

After building a pile of pedals, this is the best explanation of what is actually going on. Love the term "attempted gain", so accurate.

I’ve worked in audio electronics for 30 years and this was the best analysis of distortion pedals I’ve heard. Clear and concise. Great work man.

“Failed gain” vs Clean gain is a GREAT distinction! That’s a really great description for quickly clarifying with guitar players! Thank you!

Really excellent lecture. When I was an EE student at MIT (before switching to physics) back in the early 1980s, I took an analog electronics course with a section on op amps that included working out the transfer functions of op amp circuits with diodes. Fun stuff. But the lecturer (one of Senturia or Wedlock-we used their textbook, but I can't remember which one taught it!) didn't present it with the conceptual clarity you display here. And of course he didn't discuss guitar pedals. I had already built a distortion pedal as a teen guitarist, but didn't understand exactly what it was doing. Lightbulbs illuminated in my head during those lectures. Decades later, you provided even more insights. Thank you for sharing these lectures publicly. -Tom

I know I just learned more in this 20 min video than I have in the past month watching an assortment of other videos on RU-vid. The information was easy to digest and I'm finding that the videos and information are very linear and that the knowledge carries through. I'm 40 years old building a new life, music has been but electronics will be my new focus and attention at this mid stage of my life. Thank you for being on here and presenting us with this knowledge. I greatly appreciate it. Randy.

18:26 yes it is horrific and ridiculous, but also sounds insane. The HM-2 was used a lot by the early Swedish Death Metal scene. I think Entombed's Clandestine record is the best recording of that pedal in action and very worth listening to in my opinion

On the Boss Heavey Metal schematic, the diodes that you thought were discussing are actually acting as a gate to noise that's produced with heavy distortion/amplification. the signal has to get to a specific voltage dictated by the forward voltage of the diode to continue through the circuit. this allows hissing and buzzing to be quieter while allowing the gtr signal to go through. hope that makes more sense.

Wish we had a course like this when I was getting my EE degree. Nice work!

The two serial crossover-distortion diodes in the heavy metal distortion pedal serve a particular reason: It might act as a poor mans noise gate, so your guitar actually "shuts up" when you are not playing something. This kind of gating can be artistically useful and some pedals (mostly fuzz pedals) even have a dedicated control for this kind of gating (although I am not sure how it is typically implemented).

These videos of yours are such an amazing resource! Thx!

Hello Aaron, I have a BSE from SFSU in the late 1970's, and later earned a MSCS from Cal. Today, my engineering alma mater offers a master's program in EE/CS, but I had to do it the hard way. However, I have always been interested in both radio and electric guitar electronics, and had to figure out most of that myself. We did learn op amps and both analog and digital circuits, but the 1970s was the decade of the microprocessor, and the development of LSI to VLSI. Getting a million transistors on a wafer was all the rage, and some of my classmates went on to work for Fairchild, National, and Intel, who all recruited at my school. Now, lithography is in the nanometers. I'm so happy to be retired, but I wish I had a professor like you in my time. I enjoyed your video, as it confirmed what my ear has recently taught me. I've started playing guitar again only during the pandemic. Lately, I've been trying out pedals through my little Fender tube amp, and I've learned that all of these digital distortion pedals claim to do more than they are capable of. The Dumbler and Plexion simulators that promise "real analog sound", the Tube Screamer (without a tube!), and Distortion pedals all sound phoney to my ears. Maybe you have to grow up with the sound to like the sound of digital clipping? I grew up at the Filmore listening to the new acts of 1967 and '68 these pedals are trying to emulate. The sound is different, even if the current trace is similar. Have you done a comparison of tube vs MOS harmonic content as both go from clean into overdrive? Early in the 1980s, I knew someone at HP who demoed their new harmonic analyzer. It was an amazing box, and quite expensive -- many tens of thousands of dollars. By 2000, my home DAW had a plugin with the more resolution and better functionality done in software, than that 20 year old HP box. The beat goes on. I'll share this -- I'm grateful for the Amazon Prime free return, no questions asked policy, where I can order a few pedals for a tryout, and then send them back at the end of the month, if I don't like them. I've kept two so far -- a tuning pedal that I wish we had when I was playing professionally in the 1970's, and an analog delay pedal I gave to my grandson for Christmas last year. Sound effects are fun. Thanks again for these interesting videos, Professor.

This is a treasure trove of information for someone like me that’s just getting started in pedal design! Thanks!

One of the best videos I've seen lately, and your explanations are very clear. Put my EE degree to some use. Thank you.

Greetings Professor from Brazil! Great content, with great selection of pedals for examples. Thank you a lot.

Another great breakdown of circuit elements. Thanks.

Outstanding lecture….and FUN!

This is the exact video I spent months looking for when I first tried learning circuit design for pedals. I wish I'd found this sooner.

Wow! Amazing video. Finally a professor who can explain things in a way that is easy to understand.

Really simple and well done explanation about how a pedal distortion works. I will check out more of your videos for sure

Finally an explanation like this, on various types of overdrive/ dist! Thanks!

this was extremely helpful , thank you so much for these videos i enjoy them a lot .

super duper explanation. thank you, by heart 👏👏👏

I’m so thankful for this video! Educated me by leaps and bounds!

Thank you Dr Aaron for your very clear explanation

Horrific is in the ear of the beholder. Excellent breakdown. Thanks.

One little nitpick: the primary purpose of an overdrive pedal is to gain the signal up (often with frequency emphasis) in a mostly linear way so that the preamp section of a tube amp goes nonlinear in a desirable way. Usually these circuits also have clipping diodes like a distortion pedal, but are doing so little clipping that it would be hard to notice if you listened to the output of the overdrive pedal directly. Of course if you turn the gain up then the diodes start clipping noticeable and then you're using it as a distortion pedal at that point. As the difference is how much you turn the gain knob, a large number of pedals are suitable for both uses.

Great information, it's nice to finally find a source of more reliable information that also shows examples. And from my own state nonetheless. I'll be subscribing, I'm just learning this stuff, electrical circuitry, as a hobby.

Great lecture, wish I had a course like this at my school. Thank you for sharing.

Thank you so much for sharing this, I'm looking into building a distortion module for my DIY modular synthesizer and this has provided me with a solid foundation. Brilliant presentation, thank you again.

Great video, thanks.

Fantastic job teaching! Makes me want to go back to school:)

Great info, thanks!

I love videos and content like this

You can have this as a college class?!! Wow. That's really awesome. Those in the class should be honored to be learning this!

Thanks for this video. The animated circuit finally helped it all make sense for me. ( after deep diving into what OpAmps are actually doing ). I hope you'll consider more pedal fx circuit analysis videos. Good stuff. Thanks again.

amazing as always !!!

Two back-to-back diodes connected in series in the signal path are used to block small signals, similar to a noise gate.

A hell of an engineer! 🤘

The two diodes in series with the signal on the HM-2 are reckoned to be a noise gate - no conduction under a certain threshold.

Best video i have seen about od and ds effects so far. I am a student in engineering and I'd like to create my own guitar pedal so that'll definitely help me. Thank you very much !

I have hated distortion pedals for decades, but I heard a couple excellent players that I respect use an OCD. I bought one and love it. very versatile. Everything I have ever wanted in a gain pedal. has a bit of compression too. Interesting vid! Thak you!

I am an electronics hobbyist (at best) and this was extremely helpful. While I’ve learned a lot, much of what I’ve gathered has come in seemingly unrelated bits and pieces. This video just connected them all. Thank you!

Annnd I've found a new channel to nerd out on...

Awesome somebody else to find that uses that same circuit Sim app as I really like that app

Great video

This is really cool. I'm not a EE major (mechatronics engineering) but I built a preamp for one of my projects. It was basic, with a noninverting gain stage, low and high pass filters that I controlled with a pot. And a final output gain control. It sounded great with music but not so good with a guitar. I wish I found this video last semester.

I'm a pedal builder and EE, and I must say.... this was very well done. I wish I had something as helpful when I first started designing my pedal circuits. Most pedal builders are not EE, in fact Im the only one I know, but the community is definitely full of brilliant and creative minds. For some very interesting (and to me, impressive) circuit designs- look into the pedals of Jason Lamb. He worked for DOD in the late 80s early 90s and he went his own way.

As a nerd and a guitar player I approve of this video :D

So glad you are doing this series. I've been trying to make my own pedal board and it's been a complete failure. Hopefully this series will provide some insight!

Thanks

Very interesting

Glad to have discovered your content. At which stage do your students explore these op-amp fundamentals?

Thanks for another great lesson. Worth to say that important part of the overdrive/distortion character is toneshaping before clipping. In opamp based it is often frequency dependant gain. Sometimes diode clipping is frequency dependant (eg. BigMuff).

> "The difference between overdrive and distortion is pretty fuzzy." I see what you did here. :D

This professor rocks. Literally. I WISH my professors actually knew who Rett Shill was... You don't get how cool guitar pedals are unless you actually play. Coolest professor EVER right here kids.

Check out the curcuit for Plumes od by Earthquaker Devices - it has three different clipping settings: 1)Symmetrical diode clipping, 2)Assymetrical diode clipping, and a no-diode, op-amp only clean boost, switching the diode sets completely out. One of the best TS style pedals on the market, and quite versatile

With regards to the diodes in series in the HM-2, crossover distortion is usually pretty unpleasant, but it's mostly being masked by the other kinds of clipping going on in the circuit. Also, by blocking signals below a certain threshold the series diodes create a primitive gate of sorts. This may help reduce the noise associated with high gain.

I am the proud owner of a 62 year-old "tweed" Fender Bassman. I bought it 30 years ago and have no knowledge of the previous ownership, so I have no knowledge of any changes made to the circuitry. However, when purchased, the Presence control operated in what would nowadays be considered reverse fashion. That is, as the control was turned clockwise (up to "12"), the resulting tone got *duller*, rather than brighter. It appears as thought the intent of the control was to counteract the harmonic content coming from the output transformer, using negative feedback, and "tame" the amp into something closer to linearity.

This was fascinating, thanks! Any chance you have a video explaining why the tube-screamer type overdrives sound like they have unclipped signal mixed in?

Best source of information on the entire internet. This is amazing Is the digital distortion less fizzy? It's probably not in the scope of this class at all but it has become more popular as people search for the longer sustaining notes without a lot of fizz or losing the low end.

Very interesting video. I wish every guitar player would watch it. I've been trying to explain to them for years that every one of these pedals is basically a Tube Screamer or a Rat with different colored socks on. The series germanium diodes in the Heavy Metal pedal are probably meant to simulate the crossover distortion that one gets in a class A/B tube amplifier that is less than perfectly biased (read: practically all of them). Germanium junctions tend to be leaky and mushy which probably produces more of a swoop than a hard notch as the signal transitions between positive and negative portions of the waveform. For better or worse, overdrive/distortion pedal designers have been trying to emulate overdriven tube amplifiers from the git-go. The very first one I ever saw, an MXR Distortion+, used an ancient u741 op-amp and a "hard clipping" pair of 1N34A germanium diodes. The clipping wasn't very hard, but it was because of the sloppiness of the germanium diodes, which is exactly what MXR was trying for.

These gain to eleven !!

When I've scoped a hard clipping circuit it has some pretty square edges on the cutoff with silicon diodes. In my understanding that's how you get the harmonics that are desirable for a hard clipping circuit. I have a rat I modded with switchable silicon, germanium and LED clipping stages. Pretty common mod but great for "hearing" how the diode's clip. For a one off germanium is nice because they're so inconsistent you can dial in a bit of asymmetry (as a treat) just by testing the forward voltage of each diode and picking ones that vary to your taste.

Hi Aaron, I really enjoy your videos. Watching this made me wonder about Jurgen Haible's Wasp Filter. It has a distortion circuit that I can't quite correlate with these designs. I would love to hear your analysis of the Wasp circuit as a whole - it's quite idiosyncratic.

A distortion pedal is to push the amp into distortion whereas the fuzz is to emulate a broken speaker. The diodes are placed in the circuit in a different location.

INSTANT SUB.

I never realized that all pedal circuits are based around diodes. I always thought that BJTs were driven into saturation or cut off which caused the distortion.

That boss heavy metal distortion has such a unique sound. Behringer make a clone but for some reason it isn't the same despite all their other analogue clones being identical. I wonder if they neglected to use germanium diodes to cut cost and it changed things? Or if perhaps that oddball cross diode set up you mentioned at the end wasn't implemented correctly and changed it up.

Electronics pendant here :) Isn't the Boss DS-1 distortion stage inverting - input in to pin 2 of a single DIL op amp? Also, I found germanium diodes have a less abrupt turn on voltage or a softer "knee" than silicon diodes which give a less aggressive distortion. As an old audio electronics engineer, I still learn from each video you make! What a great resource they are.. I'm even starting to get more of the in depth math(s). Thanks!

I was reading recently that although germanium diodes have a lower forward voltage than silicon, they also have a softer curve into clipping. It's not something I've measured but it's an interesting point. I wonder what transistors wired as diodes look like on a scope?

A gain of 11 ... I see what you did there :D

@18:40 I've heard somewhere that tthe crossover distortion diode pair functions more like a "poor man's noisegate" and that might be the reasoning behind it.

1+(10K/1K) I see what you did there!

The distinctions between distortion and overdrive are fuzzy? Lol Awesome analogy.

Love that the gain in the 1st example goes up to 11. 😂

Hello Aaron, I really enjoy your videos, so knowledgable. Since you're talking about diode, how about diode based audio compressor? It was so old-fashion and compromised compressor method at that time but still in production, what's your thoughts? Thanks.

Great video! Don't forget to ask Marty McFly, he knows a lot about this.

18:36 I have found the series diodes D6/D7 don't actually provide a huge crossover distortion. Couldn't we consider them as a "diode divider" with the D9/D8 diodes? I get the impression that combined, they actually cancel out some of the effects.

11:38 -10 points to tonegeek for missing junction dots. The node at op-amp pin 2 - yeah, you can figure it out, but it is ambiguous.

Thanks for the video. I'm curious, though, why did you use triangle wave input instead of sine wave?

What's that simulator? That's so great. Awesome signal analysis!!!

The Boss HM with the germanium diods...listen to Entombeds albums Clandestine or Left hand path, and you can hear what the Boss HM pedal can do. Its called the chainsaw distortion.

Professor, i have super important question and me hope you have or can find the answer. Since you can do math with opamps. How do i measure the audio amplitude that goes above a certain "custom" threshold and measure with another op amp the difference above the max allowed and act on a VCA to remove the excess amount of amplitude. Would that something you could discus with your students to see how it could be done, how to use an op amp to do math ... on audio signals. Vertasium showed analog computers a few time and these principle although analog hardware device (wires and pulley's), but basically it was the same thing, doing math with opamps ... and converted into a video.

That Boss heavy metal pedal has crossover generating diodes probably because a tube power amplifier driven to overdrive will get less and less bias eventualy going into crossover.

I'm curious, what would happen if the N-JFET is utilized as an SD or DS diode. The gate leg as the anode, the drain leg which is connected to the source leg as the cathode.

17:34 Is this really soft clipping? It's an inverting op amp configuration and there is no resistor in series with the diodes, so the result should be hard clipping in the sense that once the diodes start to conduct, the output voltage can not go any further? Or did you just mean that the circuit topology is the same that is used for soft clipping?

When you do the analysis of the Bluesbreaker and you point out the voltage divider on the single sided PS you call it a local ground. Is that the same as bias voltage? I assumed that the 4.5V was meant to bias the signal up so that it doesn't dip into the negative. I am a bit of a noob with this stuff so please let me know if I am wrong.

I wonder if anyone gave in a Nobels odr 1 or something as an example of both hard and soft since it has both lol. Also really like crossover distortion, should check out the zvex machine I think that pedal has it deliberately, I think the lovepedal Karl is the same circuit

I’m 🌽 fusion, the input and output emitter followers on the Ibanez don’t appear to be biased at all?! That’s not an amp that’s a rectifier with gain. The boss clearly had a biasing network at the base to set the emitter at some median voltage so you can get some reasonable swing in both directions.

Yo this is the fuckin jackpot

I’m very curious why a diode bridge can be used to create a softer soft clipper. I’ve been working on better understanding Mr. Neve’s use of a diode bridge as a control voltage/soft clipper in his classic diode bridge comp/limiters - finding this technique very interesting!

How does hard clipping (back to back diodes) works ?

I'm learning a lot man, it's easy to understand and informative thank you 🙏🙏 Can u please do analytical schematic of Mountainking Megalith please? Cause that pedal is the heaviest fuzz ever in universe Crazy unstable low end texture

The catalogue for National Semiconductors, 1983, featured in an appendix a simple circuit for a fuzz box, using diodes in the feedback of an op amp. One of my students, and independently a phone technician, constructed this 'fuzz' circuit. i phoned in to National Semiconductors the fact that both these specimens turned out to be low-noisse preamps, yielding NO distortion. Whoever I struck on the phone predicably intoned 'diodes in feedback of op amp - will give distortion. They wouldn't believe me that their circuit did NOT work as claimed.

So, uhh, what does the Gamechanger plasma drive count as? Is that Hard, Soft, or Just Why clipping?

I was testing this type of circuit today, with diodes in the feedback loop, there was less drive than I expected, but I couldn’t work out how to increase it. I increased the value of Rf and lowered R1 and it helped, but what I can’t understand is why the distortion increased a lot when I removed Rf altogether, more than I could achieve playing with the values. this seemed contradictory to my understanding of op amps and increasing gain.

I love you

One time I had the idea of making a distortion effect that used two tape heads facing each other... I wonder how one would analyse that :p